avformat/dss: Don't prematurely modify context variable

avformat/dss: Don't prematurely modify context variable

The DSS demuxer currently decrements a counter that should be positive
at the beginning of read_packet; should it become negative, it means
that the data to be read can't be read contiguosly, but has to be read
in two parts. In this case the counter is incremented again after the
first read if said read succeeded; if not, the counter stays negative.

This can lead to problems in further read_packet calls; in tickets #9020
and #9023 it led to segfaults if one tries to seek lateron if the seek
failed and generic seek tried to read from the beginning. But it could
also happen when av_new_packet() failed and the user attempted to read
again afterwards.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>